fix(link-check): ignore runtime and fixture URLs

[kyle-sexton]

Closes #18

Depends on melodic-software/standards#135 and its generated standards-sync PR.

Summary

• ignore Claude Code runtime-variable links that are valid only after plugin installation expands the variable
• ignore fictional release URLs embedded in ai-briefing eval fixtures
• keep real local links and external dependency URLs checked

Why

The original --accept-timeouts compatibility failure is already fixed and the reusable workflow pin on main contains Lychee 0.24.2. Re-running the current full Markdown corpus exposed two consumer-owned false-positive classes: runtime-substituted plugin paths and fictional fixture URLs.

The remaining shared exclusions belong to the standards-managed lychee.toml: exact private raw-content inventory plus five exact public bot-blocked paths. They are implemented and boundary-tested upstream in melodic-software/standards#135 rather than copied downstream; unrelated URLs on those public hosts remain checked.

Lychee's current official documentation distinguishes URL regex exclusions (including .lycheeignore) from source-path exclusions:

• https://lychee.cli.rs/recipes/excluding-links/
• https://lychee.cli.rs/recipes/excluding-paths/
• https://lychee.cli.rs/guides/config/#excluding-links

Validation

Using this branch's .lycheeignore with the proposed standards config:

• online: 1,655 links scanned, 0 errors
• offline: 1,655 links scanned, 0 errors
• git diff --check

The scheduled hosted lane remains intentionally skipped while CI_RUNNER_POLICY=self-hosted-only; no policy change is included.